The enthalpy of formation (ΔHf) is the heat associated with the formation of one mole of a compound from its elements in their standard states. It represents the energy change when the compound is formed from its constituent elements under standard conditions.
The Hreaction is the difference between Hf, products and Hf, reactants
The relationship between exothermic formation reactions and their enthalpy of formation values is that exothermic reactions release heat energy when the compound is formed. This results in a negative enthalpy of formation value (hf) because the reaction is giving off energy.
The heat of formation (∆Hf) of carbon disulfide (CS2) can be calculated using the standard enthalpies of formation of its constituent elements as follows: ∆Hf (CS2) = ∆Hf(C) + 2∆Hf(S) − ∆Hf(CS2). The standard enthalpies of formation for carbon (C) and sulfur (S) are known values and can be used in the calculation.
The standard enthalpy change of a reaction (delta H) is related to the standard enthalpy of formation (delta Hf) of the products and reactants involved in the reaction by the equation: delta H = Σ(Products delta Hf) - Σ(Reactants delta Hf). This equation relates the enthalpy change of a reaction to the enthalpies of formation of the substances involved in the reaction.
The enthalpy change of a reaction (ΔHreaction) is related to the standard enthalpies of formation (ΔHf) of the molecules involved through Hess's Law. Hess's Law states that the overall enthalpy change for a reaction is the sum of the enthalpy changes for a series of reactions that add up to the overall reaction. The ΔHf values represent the enthalpy change when one mole of a compound is formed from its elements in their standard states.
The Hreaction is the difference between Hf, products and Hf, reactants
The relationship between exothermic formation reactions and their enthalpy of formation values is that exothermic reactions release heat energy when the compound is formed. This results in a negative enthalpy of formation value (hf) because the reaction is giving off energy.
The heat of formation (∆Hf) of carbon disulfide (CS2) can be calculated using the standard enthalpies of formation of its constituent elements as follows: ∆Hf (CS2) = ∆Hf(C) + 2∆Hf(S) − ∆Hf(CS2). The standard enthalpies of formation for carbon (C) and sulfur (S) are known values and can be used in the calculation.
The standard enthalpy change of a reaction (delta H) is related to the standard enthalpy of formation (delta Hf) of the products and reactants involved in the reaction by the equation: delta H = Σ(Products delta Hf) - Σ(Reactants delta Hf). This equation relates the enthalpy change of a reaction to the enthalpies of formation of the substances involved in the reaction.
The enthalpy of formation (Hf) refers to the change in enthalpy when one mole of a compound is formed from its elements in their standard states. The H reaction, or the enthalpy change of a specific reaction, can be calculated using the enthalpies of formation of the reactants and products involved in that reaction. According to Hess's Law, the total enthalpy change for a reaction is the sum of the enthalpy changes for individual steps, allowing for the relationship between Hf and H reaction to be quantitatively expressed in thermodynamic calculations.
The enthalpy change of a reaction (ΔHreaction) is related to the standard enthalpies of formation (ΔHf) of the molecules involved through Hess's Law. Hess's Law states that the overall enthalpy change for a reaction is the sum of the enthalpy changes for a series of reactions that add up to the overall reaction. The ΔHf values represent the enthalpy change when one mole of a compound is formed from its elements in their standard states.
2C(s) + 2H2(g) + 52.5 kJ -> C2H4
The energy change that happens when a substance forms from its elements (APEX)
2C(s) + 2H2(g) + 52.5 kJ -> C2H4
H2(g) + S(s) H2S + 20.6 kJ
C(s) + 2S(s) + 89.4 kJ --> CS2(l)
A positive enthalpy of formation (Hf) for a molecule indicates that the formation of that molecule from its constituent elements is an endothermic process, meaning it absorbs heat. This typically suggests that the molecule is less stable compared to its elements and requires energy input to form. In practical terms, a positive Hf value often signifies that the molecule is not favored to form under standard conditions.